This program represents a continuing, broadly based study of several aspects of lymphocyte immunobiology with particular emphasis on the specificity of T lymphocytes, the cellular basis of the immune response mechanism and how properties of lymphocytes become altered in neoplastic states. Specifically, the component projects deal with: 1) Functional, genetic and structural studies of T cell receptors; 2) the origin of diversity in the B cell specificity repertoire; 3) various aspects of lymphocyte function in bone marrow chimeras; 4) response to MHC alloantigens in relation to marrow transplantation; 5) the immunobiology of weak transplantation alloantigens; 6) the immunobiology of MHC expression on macrophages; 7) membrane markers of normal and neoplastic human lymphocytes; 8) molecular membrane events in lymphocyte mitogenesis; and 9) regulation of anti-MHC responses with monoclonal alloantibodies. This program is a component of The University of Pennsylvania Cancer Center program.